Mechanisms regulating macrophage-induced nitric oxide production by spontaneously transformed hamster fibroblasts

Nitric oxide has been implicated as an important effector molecule involved in tumor cell growth and cytotoxicity. In these studies we examined mechanisms regulating nitric oxide production by hamster tumor cells. Cocultures of hamster alveolar macrophages (HAM) and spontaneously transformed hamster embryonic fibroblasts (STHE cells) produced significant quantities of nitric oxide in response to lipopolysaccharide (LPS). Culture supernatants from HAM treated with LPS also stimulated nitric oxide production by STHE cells, whereas tumor cell culture supernatants had no effect on HAM. These data, together with the findings that paraformaldehyde treatment of STHE cells, but not macrophages, completely abrogated nitric oxide production in the cocultures demonstrate that the tumor cells were the source of this mediator. In contrast to STHE cells, STHE-83/20 cells, a highly malignant variant, did not produce nitric oxide in response to HAM or HAM culture supernatants even in the presence of LPS. Both anti-tumor necrosis factor-alpha (TNF-alpha) and anti-interleukin-1alpha (IL-1alpha) antibodies inhibited HAM-induced nitric oxide production by STHE cells. However, the kinetics of their effects were different. Moreover, although the nitric oxide stimulating activity in HAM culture supernatants was abrogated by anti-TNF-alpha antibody, it was only minimally reduced by anti-IL-1alpha antibody. These data demonstrate that TNF-alpha and IL-1alpha play distinct roles in induction of nitric oxide synthesis in STHE cells. HAM were also found to suppress proliferation of STHE cells, an effect that was inhibited by anti-TNF-alpha antibody, but not NG-monomethyl-L-arginine, which blocks nitric oxide synthase. Abrogation of macrophage-induced cytostasis in STHE cells by anti-TNF-alpha antibody was associated with decreased nitric oxide production. Thus TNF-alpha released by macrophages may indirectly activate STHE cells for nitric oxide synthesis by suppressing tumor cell proliferation.